Water is an essential commodity for survival of life on these plants. The circulation of water and moisture between living organism, atmosphere and earth is called as water cycle or the hydrological cycle. It is also described as movement and existence of water on, in and above the earth.

This cycle includes:

(i)           The evaporation of water directly from water bodies like lakes, seas, oceans, soil surface etc. it also includes water from transpiring plant. The rate of evaporation depends upon temperature and light of that place.

(ii)         The water vapour rises up into the air where low temperature causes it to condense into clodus which precipitates as rain, snow or hail etc.

(iii)       Water in the form of snow stays in the icy mountain caps and glacier for years but in warmer climate snow melts and water comes into rivers which gradually enter into sea.

(iv)       Water in the form of rain make either runoff surface water and enters into rivers or penetrate deep inside earthy crust and form ground water source. Sometime this water seeps back into surface water bodies (ponds, lakes, oceans etc.) or may come out as fresh water spring.

This way water cycle continues forever.